Solid epoxidation catalyst and preparation

ABSTRACT

The stability of a noble metal/titanium zeolite catalyst is improved by elevated temperature calcination with an oxygen containing gas; the catalyst is useful in epoxidation involving the reaction of olefin, oxygen and hydrogen.

This is a division of appl. Ser. No. 09/731,565, filed Dec. 7, 2000,U.S. Pat. No. 6,281,369, issued Aug. 28, 2001.

FIELD OF THE INVENTION

This invention relates to the preparation of a novel epoxidationcatalyst comprised of a titanium zeolite catalyst which has beenmodified with a noble metal such as palladium, which catalyst hasenhanced stability, and to the use of the catalyst for the production ofoxirane compounds such as propylene oxide.

BACKGROUND OF THE INVENTION

Oxiranes constitute an important class of chemical intermediates usefulfor the preparation of polyether polyols, glycols, glycol ethers,surfactants, functional fluids, fuel additives and the like. Manydifferent methods for synthesizing oxiranes from the correspondingolefins have been described in the literature. A Japanese patentapplication assigned to the Tosoh Corporation and published in 1992(Kokai No. 4-352771) proposed making propylene oxide by reactingpropylene, hydrogen and oxygen using a catalyst comprising a Group VIIImetal and a crystalline titanosilicate. Improvements to or variations ofthis basic process were subsequently described in the followingpublished patent applications: WO 97/25143, DE 19600709, WO 96/02323, WO97/47386, WO 97/31711, JP H8-269030, JP H8-269029, U.S. Pat. Nos.6,005,123, 6,008,388 and 5,646,314.

As with any chemical process, it would be desirable to attain stillfurther improvements in epoxidation methods of this type. In particular,extending the useful life of the catalyst would significantly enhancethe commercial potential of such methods. A problem has been that thenoble metal tends to be leached or otherwise lost from the catalystduring use which results in loss of activity and selectivity.Additionally, loss of noble metal imposes an economic penalty which mayrender the process uneconomic.

SUMMARY OF THE INVENTION

The present invention provides a process for the preparation and use ofa catalyst comprised of a titanium zeolite and a noble metalcharacterized in that the catalyst has improved stability and resistanceto loss from the zeolite during use, and to the use of same inepoxidations.

DETAILED DESCRIPTION OF THE INVENTION

The catalysts of the present invention are comprised of a titaniumzeolite and a noble metal (preferably an element of Group VIII of thePeriodic Table). Suitable zeolites are those crystalline materialshaving a porous molecular sieve structure with titanium atomssubstituted in the framework. The choice of zeolite employed will dependupon a number of factors, including the size and shape of the olefin tobe epoxidized. For example, it is preferred to use a relatively smallpore titanium zeolite such as a titanium silicalite if the olefin is alower aliphatic olefin such as ethylene, propylene, or n-butene. Wherethe olefin is propylene, the use of a TS-1 titanium silicalite isespecially advantageous. For a bulky olefin such as cyclohexene, alarger pore titanium zeolite such as a titanium zeolite having astructure isomorphous with zeolite beta may be preferred.

The titanium-containing zeolites useful as catalysts in the epoxidationstep of the process comprise the class of zeolitic substances whereintitanium atoms are substituted for a portion of the silicon atoms in thelattice framework of a molecular sieve. Such substances are known in theart.

Particularly preferred titanium-containing zeolites include the class ofmolecular sieves commonly referred to as titanium silicalites,particularly “TS-1” (having an MFI topology analogous to that of theZSM-5 aluminosilicate zeolites). “TS-2” (having an MEL topologyanalogous to that of the ZSM-11 aluminosilicate zeolites), and “TS-3”(as described in Belgian Pat. No. 1,001,038). Also suitable for use arethe titanium-containing molecular sieves having framework structuresisomorphous to zeolite beta, mordenite, ZSM-48, ZSM-12, and MCM-41. Thetitanium-containing zeolite preferably contains no elements other thantitanium, silicon and oxygen in the lattice framework, although minoramounts of boron, iron, aluminum, and the like may be present. Othermetals such as tin or vanadium may also be present in the latticeframework of the zeolite in addition to the titanium, as described inU.S. Pat. Nos. 5,780,654 and 5,744,619.

Preferred titanium-containing zeolite catalysts suitable for use in theprocess of this invention will generally have a compositioncorresponding to the following empirical formula xTiO₂. (1−x)SiO₂ wherex is between 0.0001 and 0.500. More preferably, the value of x is from0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of thezeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1to 60:1). The use of relatively titanium-rich zeolites may also bedesirable. The zeolite may or may not contain extra framework titanium.

As an essential aspect of the present invention, the catalyst comprisesa noble metal supported on the above-described supports.

While any of the noble metals can be utilized (i.e., gold, silver,platinum, palladium, iridium, ruthenium, osmium), either alone or incombination, palladium is particularly desirable. Typically, the amountof noble metal present in the catalyst will be in the range of from 0.01to 5 weight percent, preferably 0.05 to 2 weight percent. The manner inwhich the noble metal is incorporated into the catalyst is a criticalfeature of the invention.

The titanium silicalite used in the present invention is prepared byknown procedures. An important feature is that the silicalite besubjected to an oxidative calcination as with air at elevatedtemperature, eg. 300 to 850° C., illustratively 550° C., in accordancewith known procedures prior to use in accordance with the invention. Thecalcination is carried out until substantially complete removal oforganic residues is accomplished. Thorough pre-washing and oxidativecalcination procedures are described, for example in JP H-269029 and JPH-269030.

The titanium silicalite washing and calcination is carried out so as toremove essentially all of the residues of materials such as templatingagents and the like used in the silicalite preparation, especiallyammonium-type materials.

The calcined silicalite essentially free of residues is then treated asby ion exchange or impregnation procedures in order to incorporate thedesired noble metal into the silicalite in appropriate amounts. Of theprocedures, ion exchange is preferred with subsequent essentiallycomplete removal of anionic residues from the resulting catalyst.Impregnation procedures can be used as is described herein later.

Removal of essentially all residues from the noble metal containingsupport is important and is conventionally accomplished by water washingand filtering techniques. Multiple washing and filtering steps areespecially preferred. Preferably the noble metal/titanium silicalitecatalyst is then dried by gentle heating, for example under vacuum.

A critical step in the preparation procedure is oxidative calcination ofthe noble metal/titanium silicate catalyst. Whereas prior art such as JPH8-269029 and JP H8-269030 teaches reduction of the noble metal/silicatecatalyst, eg. 90° C. with a H₂/N₂ reducing gas, before use inepoxidation reactions, it has now been found that such prior catalystsare prone to rapid leaching of noble metal during epoxidation use thusseverely limiting the practical utility of such catalysts.

It has now been found that the oxidation calcination of the noblemetal/silicate catalyst results in the formation of a useful catalystcomposition having greatly improved stability as against noble metalloss and thus greatly improved utility in the production of oxiraneproduct such as propylene oxide.

The oxidative calcination is carried out at temperatures of at least150° C. for illustratively 10 minutes to 24 hours. Calcinationtemperature in the range 150-650° C., preferably 250-600° C., and mostpreferably 300-550° C. are employed. The calcination gas is preferablyair by reason of cost and availability although other mixtures of oxygenand inert gas can be used. Generally during the calcination it isadvantageous to ramp the temperature up at the rate of 0.5-10° C.,preferably 1-5° C./min to the desired upper temperature.

The above preparation markedly reduces noble metal loss during use ofthe catalyst in epoxidation reactions as will be demonstrated inexperimental results herein after presented.

Additional improvements are also achieved where prior to or duringepoxidation the catalyst is contacted with solutions buffered toslightly acid to basic pH. The preferred pH range is 5-8, preferably6-7.5. See, for example, U.S. Pat. No. 5,646,314. Especiallyadvantageous is the use of sodium and/or potassium salt bufferedsolutions. Excellent results are also achieved with calcium andmagnesium salt containing solutions. Other Group I a and II a salts canbe used as can compounds such as triphenyl phosphine. The combination ofthe calcination and contact with the buffered solution gives bestresults.

The olefin to be epoxidized can be any organic compound containing atleast one site of ethylene unsaturation (i.e., at least onecarbon—carbon double bond). The olefin can be aliphatic, aromatic orcycloaliphatic in character and may have either a linear or branchedstructure, with the site(s) of ethylenic unsaturation being terminaland/or internal. The olefin preferably contains 2-30 carbon atoms; theprocess of the invention is particularly suitable for epoxidizing C₂-C₆mono-olefins. More than one double bond may be present, as in a diene ortriene for example. The olefin may be a hydrocarbon (i.e., contain onlycarbon and hydrogen atoms) or may contain functional groups such ashalide, carboxyl, hydroxyl, ether, carbonyl, cyano, or nitro, groups orthe like.

Typical examples of suitable olefins include ethylene, propylene,1-butene, cis-and trans-2-butene, 1,3-butadiene, pentenes, isoprene,hexenes, octenes, nonenes, decenes, undecenes, dodecenes, cyclopentene,cyclohexene, dicyclopentadiene, vinylcylohexane, vinyl cyclohexene,allyl chloride, allyl alcohol, methallyl chloride, methallyl alcohol,alkyl acrylates and methacrylates, unsaturated fatty acids and estersthereof, styrene, α-methylstyrene, divinylbenzene, indene and stilbene.Mixtures of olefins may, of course, be utilized if so desired. Theprocess of this invention is especially useful for converting propyleneto propylene oxide.

The process of the invention may be suitably conducted under thereaction conditions (e.g., temperature, pressure, reactant ratios)described in the following published patent applications: WO 96102323,WO 97/25143, DE 19600709, WO 97/31711, WO 97/47386, JP 4-352771, JPH8-269029, and H8-269030.

The amount of catalyst used may be determined on the basis of the molarratio of the titanium contained in the titanium zeolite to the olefinthat is supplied per unit of time. Typically, sufficient catalyst ispresent to provide a titanium/olefin fed ratio of from 0.00001 to 0.1per hour. The time required for the epoxidation may be determined on thebasis of the gas hourly space velocity, i.e., the total volume ofolefin, hydrogen, oxygen and carrier gas(es) per hour per unit ofcatalyst volume (abbreviated as GHSV). A GHSV in the range of 0.1 to10,000 hr⁻¹ is typically satisfactory.

Depending on the olefin to be reacted, the epoxidation according to theinvention can be carried out in the liquid phase, vapor phase, or in thesupercritical phase. When a liquid reaction medium is used, as ispreferred, the catalyst is preferably in the form of a suspension or asin fixed bed mode. The process may be performed using a continuous flow,semi-batch or batch mode of operation.

If epoxidation is carried out in the liquid phase, it is advantageous towork at a pressure of 1-100 bars and in the presence of one or moresolvents. Suitable solvents include, but are not limited to, loweraliphatic alcohols such as methanol, ethanol, isopropanol, andtert-butanol, or mixtures thereof, and water. Fluorinated alcohols canbe used. It is also possible to use mixtures of the cited alcohols withwater. A mixture of water and methanol is preferred as solvent;hydrocarbons such as propane and/or propylene can be used as can carbondioxide. Epoxidation according to the invention is carried out at atemperature effective to achieve the desired olefin epoxidation,preferably at temperatures in the range of 0-125° C. (more preferably,20-80° C.). The molar ratio of hydrogen to oxygen can usually be variedin the range of H₂:O₂=1:10 to 5:1 and is especially favorable at 1:5 to1:1. The molar ratio of oxygen to olefin can be 3:1 or more butpreferably is 1:1 to 1:20, and most preferably 1:1.5 to 1:10. Relativelylow O₂ to olefin molar ratios (e.g., 1:1 to 1:3) may be advantageous forcertain olefins. As the carrier gas, any desired inert gas can be used.The molar ratio of olefin to carrier gas is then usually in the range of50:1 to 1:50, and especially 20:1 to 1:1.

As the inert carrier gas, noble gases such as helium, neon, argon,krypton, and xenon are suitable in addition to nitrogen and carbondioxide. Saturated hydrocarbons with 1-8, especially 1-6, and preferablewith 1-4 carbon atoms, e.g., methane, ethane, propane, and n-butane, arealso suitable. Nitrogen and saturated C₁-C₄ hydrocarbons are thepreferred inert carrier gases. Mixtures of the listed inert carriergases can also be used.

Specifically in the epoxidation of propylene according to the invention,propane can be supplied in such a way that, in the presence of anappropriate excess of carrier gas, the explosive limits of mixtures ofpropylene, propane, hydrogen, and oxygen are safely avoided and thus noexplosive mixture can form in the reactor or in the feed and dischargelines.

EXAMPLES

As used herein, POE refers to propylene oxide and compounds derived frompropylene oxide such as propylene glycol (PG), methoxy propanol,dipropylene glycol, tripropylene glycol, acetol, dipropylene glycolmethyl ether, triproylene glycol methyl ether, and the like.

Selectivities are mols of product divided by the mols of reactantconsumed multiplied by 100. Thus propylene based selectivity to POE(SPPOE) is the mols of POE divided by the mols of propylene consumedmultiplied by 100. The hydrogen based selectivity to POE (SHPOE) is themols of POE divided by the mols of hydrogen consumed multiplied by 100.The oxygen based selectivity to POE (SOPOE) is the mols of POE formeddivided by the mols of oxygen consumed multiplied by 100.

Example 1

Catalyst A was made by ion exchanging Pd(II) from an aqueous solution oftetraamine palladium (II) chloride in excess ammonia on calcined TS-1,the TS-1 being added batchwise to the palladium solution. The mixturewas agitated for 1 hour, filtered and the solid phase was washed withdeionized water three times. The solid was dried at 50° C. in a vacuumoven, followed by calcination in air by ramping to 500° C. at 2° C./minand holding for 4 hours. The final catalyst had 0.45 wt % Pd and 2.01 wt% Ti.

1 gm of catalyst A was slurried in 100 cc of a pH buffer consisting ofan aqueous solution of potassium dihydrogen phosphate and sodiumhydroxide for 46.5 hrs at 60° C. and 1.5 psig with 1000 RPM stir baragitation, 100 cc/min of gas feed with 9.96 vol % propylene, 3.73 vol %oxygen and 3.77 vol % hydrogen was fed. The mean POE rate was 0.00504gmPO/gm cat hr, the mean propylene-based selectivity to POE was 68%, themean oxygen-based selectivity to POE was 4% and the mean hydrogen-basedselectivity to POE was 2%. The POE formed was 61% PO and 39% ring-openproducts, mainly PG. The liquid phase pH was 6.4 throughout. The Pd lossfrom the catalyst computed as −2.9% i.e. an apparent gain of 2.9%.

Example 2

Catalyst preparation B used Pd(II) trifluoroacetate in dilute aqueoussolution, added continuously to a well-mixed aqueous slurry ofair-calcined powdered 0.2 micron crystallite diameter TS-1 over 16hours, the palladium(II) ions exchanging with the protons on the TS-1.The solid was filtered from the liquid and resuspended in deionizedwater and refiltered, three times. The material was dried under vacuumat 50° C. and calcined in air by ramping to 500° C. at 2° C./min andholding for 4 hours. The catalyst was then slurried in an aqueoussolution of monosodium dihydrogen phosphate for 24 hours, filtered,resuspended in fresh deionized water and filtered. It was then dried at50° C. under vacuum. The final catalyst was 0.1365 wt % Pd and 1.525 wt% Ti.

1 gm of catalyst B was run semi-continuously by slurrying in 100 cc ofdeionized water for 46.5 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 3.16 vol % propylene, 3.54vol % oxygen and 4.14 vol % hydrogen. The mean POE rate was 0.0080gmPO/gm cat hr, the mean propylene-based selectivity to POE was 99%, themean oxygen-based selectivity to POE was 45% and the mean hydrogen-basedselectivity to POE was 28%. The POE formed was 21% PO and 79% ring-openproducts, almost exclusively PG. The liquid phase pH fell from 5.23 to3.22 during the run. The Pd loss from the catalyst computed as −7.7%i.e. an apparent gain of 7.7%.

Example 3

Catalyst C was made by ion exchanging Pd(II) from an aqueous solution oftetraamine palladium (II) dinitrate in excess ammonia to calcined TS-1,the TS-1 being added batchwise to the palladium solution. The mixturewas agitated for 24 hours at 80° C., filtered, and the solid phase waswashed with deionized water three times. The solid was dried at 60° C.in a vacuum oven, followed by calcined in air by ramping to 500° C. at2° C./min and holding for 4 hours. The final catalyst had 0.19 wt % Pdand 0.89 wt % Ti.

0.5 gm of catalyst C was run semi-continuously by slurring in 100 cc ofa pH buffer consisting of an aqueous solution of 0.1M potassiumdihydrogen phosphate and 0.1M potassium hydroxide for 1 hour at 60° C.and 1.5 psig with 1000 RPM stir bar agitation, 100 cc/min of gas feedwith 10 vol % oxygen and 4.0 vol % hydrogen. The mean POE rate was0.0032 gmPOE/gm cat hr, the mean propylene-based selectivity to POE was24%, the mean oxygen-based selectivity to POE was 2% and the meanhydrogen-based selectivity to POE was 1%. The POE formed was 58% PO and42% ring-open products, mainly PG. The liquid phase pH was 6.4throughout. The Pd loss from the catalyst computed as −8.1% i.e. anapparent gain of 8.1%.

Example 4

0.5 gm of catalyst C was run semi-continuously by slurrying in 100 cc ofa pH buffer consisting of an aqueous solution of 0.01M potassiumdihydrogen phosphate and 0.01M potassium hydroxide for 99 hours at 60°C. and 1.5 psig with 1000 RPM stir bar agitation, 100 cc/min of gas feedwith 10 vol % propylene, 4.0 vol % oxygen and 4.0 vol % hydrogen. Themean POE rate was 0.0030 gmPOE/gm cat hr, the mean propylene-basedselectivity to POE was 82.3%, the mean oxygen-based selectivity to POEwas 4% and the mean hydrogen-based selectivity to POE was 3%. The POEformed was 77% PO and 28% ring-open products, mainly PG. The liquidphase pH was 6.7 throughout. The Pd loss from the catalyst computed as−8.1% i.e. an apparent gain of 8.1%.

Example 5

(Comparative)

Catalyst E was made by using an impregnation technique to add Pd(II)from an aqueous solution of tetraamine palladium (II) dinitrate inexcess ammonia to calcined TS-1, the TS-1 being added batchwise to thepalladium solution. The mixture was agitated for 24 hours at 80° C. androtovaped. The solid was dried at 60° C. in a vacuum oven, followed byheating in N₂ by ramping to 150° C. at 2° C./min and holding for 4hours. The final catalyst had 0.51 wt % Pd and 0.92 wt % Ti.

1.0 gm of catalyst E was run semi-continuously by slurrying in 100 cc ofdistilled water for 105 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 10 vol % propylene 4.0 vol %hydrogen. The mean POE rate was 0.0174 gmPOE/gm cat hr, the meanpropylene-based selectivity to POE was 88%, the mean oxygen-basedselectivity to POE was 27% and the mean hydrogen-based selectivity toPOE was 15%. The POE formed was 32% PO and 68% ring-open products,mainly PG. The liquid phase pH was 5.2 throughout. The Pd loss from thecatalyst computed as 58.8%.

Example 6

(Comparative)

Catalyst F was made by impregnation of aqueous Pd(II) tetraaminedinitrate in excess ammonia onto calcined TS-1, the TS-1 being batchwiseadded to the palladium solution. It was heated to 80° C. for 16 hoursand then the water was stripped under vacuum at 50° C. and then thesolid was dried at 60° C. under vacuum for 24 hours. The catalyst wasthen heated to 150° C. for 4 hours in flowing nitrogen. The finalcatalyst had 0.55 wt % Pd and 2.1 wt % Ti.

0.5 gm of catalyst F was run semi-continuously by slurrying in 100 cc ofwater for 54 hours at 60° C. and 3 psig, with 1000 RPM stir baragitation, 100 cc/min of gas feed with 9.80 vol % propylene, 3.87 vol %oxygen and 4.21 vol % hydrogen. The solution pH fell to 4.08 at theclose of the run. The mean POE rate was 0.0405 gm PO/gm cat hr, the meanpropylene-based selectivity to POE (SPPOE) was 96%, the meanoxygen-based selectivity to POE (SOPOE) was 49% and the meanhydrogen-based selectivity to POE (SHPOE) was 26%. The POE was 66% and34% ring-open products, mainly PG. The Pd loss from the catalyst wasmeasured as 63%.

Example 7

Catalyst G was made by ion exchanging Pd(II) from an aqueous solution oftetraamine palladium (II) dinitrate in excess ammonia to calcined TS-1,the TS-1 being added batchwise to the palladium solution. The mixturewas agitated for 24 hours to 80° C., filtered and the solid phase waswashed with deionized water three times. The solid was dried at 60° C.in a vacuum oven, followed by calcination in air by ramping to 500° C.at 2° C./min and holding for 4 hours. The final catalyst had 0.53 wt %Pd and 0.91 wt % Ti.

1.0 gm of catalyst G was run semi-continuously by slurrying in 100 cc ofdistilled water for 125 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 10 vol % propylene, 4.0 vol %oxygen and 4.0 vol % hydrogen. The mean POE rate was 0.0041 gmPOE/gm cathr, the mean propylene-based selectivity to POE was 54%, the meanoxygen-based selectivity to POE was 4% and the mean hydrogen-basedselectivity to POE was 2%. The POE formed was 2% PO and 98% ring-openproducts, mainly PG. The liquid phase pH was 3.5 throughout. The Pd lossfrom the catalyst computed as 17%.

Example 8

Catalyst H was made by ion exchanging Pd(II) from an aqueous solution oftetraamine palladium (II) dinitrate in excess ammonia to calcined TS-1,the TS-1 being added batchwise to the palladium solution. The mixturewas agitated for 24 hours at 80° C., filtered and the solid phase waswashed with deionized water three times. The solid was dried at 60° C.in a vacuum oven, followed by calcination in air by ramping to 500° C.at 2° C./min and holding for 4 hours. The final catalyst had 0.32 wt %Pd and 0.90 wt % Ti.

0.5 gm of catalyst H was run semi-continuously by slurrying in 100 cc ofdistilled water for 99 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 10 vol % propylene, 4.0 vol %oxygen and 4.0 vol % hydrogen. The mean POE rate was 0.0105 gmPOE/gm cathr, the mean propylene-based selectivity to POE was 65%, the meanoxygen-based selectivity to POE was 6% and the mean hydrogen-basedselectivity to POE was 3%. The POE formed was 5% PO and 95% ring-openproducts, mainly PG. The liquid phase pH was 3.8 throughout. The Pd lossfrom the catalyst computed as 22%.

Example 9

0.5 gm of catalyst C was run semi-continuously by slurrying in 100 cc ofdistilled water for 126 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 10% vol propylene, 4.0 vol %oxygen and 4.0 vol % hydrogen. The mean POE rate was 0.0063 gmPOE/gm cathr, the mean propylene-based selectivity to POE was 88%, the mean oxygenbased selectivity to POE was 6% and the mean hydrogen-based selectivityto POE was 3%. The POE formed was 7% PO and 92% ring-open products,mainly PG. The liquid phase pH was 4.0 throughout. The Pd loss from thecatalyst computed as 21%.

Example 10

3 gm of catalyst D were run semi-continuously by slurrying in 100 cc ofwater for 93 hours at 60° C. and 3 psig with 1000 RPM stir baragitation, 100 cc/min of gas feed with 9.86 vol % propylene, 3.77 vol %oxygen and 4.38 vol % hydrogen. The solution pH fell from 5.88 to 2.71during the run. The mean POE rate was 0.0011 gm PO/gm cat hr, the meanpropylene-based selectivity to POE (SPPOE) was 6%, the mean oxygen-basedselectivity to POE (SOPOE) was 2% and the mean hydrogen-basedselectivity to POE (SHPOE) was 1%. The POE was 29% PO and 71% ring-openproducts, mainly PG. The Pd loss from the catalyst was measured as 8.4%.

Example 11

(Comparative)

1.0 gm of catalyst G was run semi-continuously by slurrying in 100 cc ofdistilled water for 138 hours at 60° C. and 1.5 psig with 1000 RPM stirbar agitation, 100 cc/min of gas feed with 10 vol % propylene, 4.0 vol %oxygen and 4.0 vol % hydrogen. The mean POE rate was 0.0090 gmPOE/gm cathr, the mean propylene-based selectivity to POE was 88%, the meanoxygen-based selectivity to POE was 40% and the mean hydrogen-basedselectivity to POE was 45%. The POE formed was 37% PO and 63% ring-openproducts, mainly PG. The liquid phase pH was 5.8 throughout. The Pd lossfrom the catalyst computed as 68%.

Example 12

(Comparative)

Catalyst D was made by ion exchanging Pd(II) ions from an aqueoussolution of tetraamine palladium (II) chloride in excess ammonia tocalcined TS-1, the TS-1 being batchwise added to the palladium solution.The mixture was agitated for 1 hour, filtered and the solid phase waswashed with deionized water three times. The solid was dried at 50° C.in a vacuum oven. The final catalyst had 0.49 wt % Pd and 1.86 wt % Ti.

1.5 gm of catalyst D were run semi-continuously by slurrying in 100 ccof water for 46.5 hours at 60° C. and 3 psig, with 1000 RPM stir baragitation, 100 cc/min of gas feed with 9.17 vol % propylene, 3.93 vol %oxygen and 3.91 vol % hydrogen. The reactor solution pH fell from 8.07to 4.05 during the run. The mean POE rate was 0.0185 gm PO/gm cat hr themean propylene-based selectivity to POE (SPPOE) was 92%, the meanoxygen-based selectivity to POE (SOPOE) was 44% and the meanhydrogen-based selectivity to POE (SHPOE) was 27%. The POE was 41% POand 59% ring-open products, mainly PG. The Pd loss from the catalyst wasmeasured as 60%.

Example 13

Catalyst I was made by impregnation of aqueous Pd(II) tetraaminedinitrate in excess ammonia onto calcined evacuated TS-1, the TS-1 beingbatchwise added to the palladium solution. It was heated to 80° C. for16 hours and then the water was stripped under vacuum at 50° C. and thenthe solid was dried at 60° C. under vacuum for 24 hours. The catalystwas then heated to 150° C. for 4 hours in flowing nitrogen and then thematerial was calcined in air by ramping to 500° C. at 2° C./min andholding for 4 hours. The final catalyst had 0.60 wt % Pd and 1.96 wt %Ti.

3 gm catalyst I were run semi-continuously by slurrying in 100 cc ofwater for 46.5 hours at 60° C. and 3 psig, with 1000 RPM stir baragitation, 100 cc/min of gas feed with 10.1 vol % propylene, 3.9 vol %oxygen and 4.9 vol % hydrogen. The solution pH fell from 5.29 to 3.71during the run. The mean POE rate was 0.0335 gm PO/gm cat hr, the meanpropylene-based selectivity to POE (SPPOE) was 67%, the meanoxygen-based selectivity to POE (SOPOE) was 20% and the meanhydrogen-based selectivity to POE (SHPOE) was 9%. The POE was 3% PO and97% ring-open products, mainly PG. The Pd loss from the catalyst wasmeasured as 12%.

Example 14

(Comparative)

1.0 gm of catalyst E was run semi-continuously by slurrying in 100 cc ofa pH buffer consisting of an aqueous solution of 0.1M potassiumdehydrogen phosphate and 0.1M potassium hydroxide for 1 hour at 60° C.and 1.5 psig with 1000 RPM stir bar agitation, 100 cc/min of gas feedwith 10 vol % propylene, 4.0 vol % oxygen and 4.0 vol % hydrogen. Themean POE rate was 0.0052 gmPOE/gm cat hr, the mean propylene-basedselectivity to POE was 94%, the mean oxygen-based selectivity to POE was6% and the mean hydrogen-based selectivity to POE was 3%. The POE formedwas 80% PO and 20% ring-open products, mainly PG. The liquid phase pHwas 6.7 throughout. The Pd loss from the catalyst computed as 13.7%.

Example 15

(Comparative)

0.5 gm of catalyst F was run semi-continuously by slurrying in 100 cc ofa pH buffer consisting of an aqueous solution of potassium dihydrogenphosphate and sodium hydroxide for 70.5 hours at 60° C. and 3 psig, with1000 RPM stir bar agitation, 100 cc/min of gas feed with 9.57 vol %propylene, 3.77 vol % oxygen and 3.74 vol % hydrogen. The solution pHwas 6.5 throughout. The mean POE rate was 0.026 gm PO/gm cat hr, themean propylene-based selectivity to POE (SPPOE) was 95%, the meanoxygen-base selectivity to POE (SOPOE) was 21% and the meanhydrogen-based selectivity to POE (SHPOE) was 12%. The POE was 84% POand 16% ring-open products, mainly PG. The Pd loss from the catalyst wasmeasured as 4.8%.

Example 16

(Comparative)

0.5 gm of catalyst F was run semi-continuously by slurrying in 100 cc ofa pH buffer consisting of an aqueous solution of potassium dihydrogenphosphate and sodium hydroxide for 54 hours at 60° C. and 3 psig, with1000 RPM stir bar agitation, 100 cc/min of gas feed with 8.84 vol %propylene, 3.84 vol % oxygen and 4.05 vol % hydrogen. The solution pHwas 5.5 throughout. The mean POE rate was 0.022 gm PO/gmn cat hr, themean oxygen-based selectivity to POE (SPPOE) was 94%, the meanoxygen-based selectivity to POE (SOPOE) was 14% and the meanhydrogen-based selectivity to POE (SHPOE) was 6%. The POE was 79% PO and21% ring-open products, mainly PG. The Pd loss from the catalyst wasmeasured as 15%.

Examples 1-4 demonstrate the outstanding stability of catalysts preparedin accordance with most preferred practice of the invention where thebuffered epoxidation solution was used in conjunction the catalystpreparation.

Comparative Examples 5, 6, 11 and 12 demonstrate the high rate of noblemetal loss from catalysts not prepared by the invention and used innon-buffered epoxidation solution.

Examples 7, shows use of catalysts prepared in accordance with theinvention and used in non-buffered epoxidation solution. Results arebetter than those of Examples 5, 6, 11 and 12 but inferior to those ofExamples 1-4.

Comparative Examples 14, 15, and 16 illustrate that catalysts notprepared in accordance with the invention have a higher loss of noblemetal even in buffered epoxidation solution as compared to similar runswith catalysts prepared by the invention.

We claim:
 1. In a process for the preparation of a noble metal andtitanium zeolite catalyst useful for the epoxidation of olefins, whereinthe titanium zeolite catalyst contains no elements other than titanium,silicon and oxygen in the lattice framework, the improvement whichcomprises calcining the catalyst at temperatures above 150° C. in anoxygen containing atmosphere.
 2. The process of claim 1 wherein thecalcination temperature is 250-600° C.
 3. The process of claim 1 whereinthe calcination temperature is 300-550° C.
 4. The process of claim 1wherein the noble metal is palladium.
 5. The process of claim 1 whereinthe titanium zeolite is precalcined at 300° C. to 850° C. prior toaddition of noble metal.
 6. A catalyst prepared by the process of claim1.